1. Field of the Invention
This invention relates to a process for regenerating a bleaching-fixing solution, and more specifically, to a process for regenerating a bleaching-fixing solution by electrolysis while maintaining the solution in contact with an oxygencontaining gas.
2. Description of the Prior Art
The processing of a silver halide color photographic material includes the two steps of color development and silver removal. When a color photographic material is subjected to color development, the exposed silver halide is reduced to silver, and simultaneously the oxidized color developing agent (oxidized aromatic primary amine) reacts with a coupler to form a dye image. The silver formed by the development is oxidized with a bleaching agent (oxidizing agent) in the silver removal step to convert the silver to silver halide which is then dissolved with a silver halide solvent and thus removed.
As described, the silver removal step includes the two steps of bleaching and fixing. In photographic processing methods now in actual use, the bleaching step and the fixing step are either separately carried out, or performed simultaneously as disclosed in U.S. Pat. No. 3,582,322.
An iron (III) chelate compound present in the bleaching-fixing solution oxidizes the developed silver during processing, and is itself reduced to a divalent iron salt. On the other hand, the silver ion formed by oxidation is dissolved by the fixing agent. Hence, as the processing is continued, the iron (II) salt and a silver complex salt gradually accumulate in the bleaching-fixing solution. Thus, the activity of the bleaching-fixing solution is decreased, and fatigue is exhibited. In order to prevent this, the bleaching-fixing solution is replenished with a suitable amount of a fresh supply of the bleaching-fixing solution at the time of development, and the used solution is discharged by an overflow from the tank. To discard the used solution directly is undesirable not only from the viewpoint of environmental pollution but also from an economical viewpoint. It would be very advantageous if the activity of the solution could be revived and the solution reused.
A method for recovering silver from a fixing bath can generally be utilized for recovering silver from a bleaching-fixing bath. Known methods for silver recovery in the photographic industry include:
(1) A METHOD WHICH COMPRISES ADDING AN AGENT FOR FORMING AN INSOLUBLE SILVER SALT TO BE USED BLEACHING-FIXING SOLUTION (SILVER PRECIPITATION METHOD);
(2) A METHOD WHICH COMPRISES CONTACTING THE USED BLEACHING-FIXING SOLUTION WITH A METAL HAVING A HIGHER IONIZING TENDENCY THAN SILVER (METAL SUBSTITUTION METHOD)
(3) A METHOD WHICH COMPRISES DEPOSITING SILVER ON A CATHODE IN AN ELECTROLYTIC CELL (ELECTROLYZING METHOD); AND
(4) A METHOD WHICH COMPRISES UTILIZING AN ION EXCHANGE RESIN (ION EXCHANGE METHOD).
The details of these methods are described in M. L. Schreibe "Present Status of Silver Recovery in Mothion-Picture Laboratories", J. SMPTE, Vol. 74, pages 505-514, (1965).
Various methods have been suggested heretofore on the oxidation of the iron (II) ion formed in the bleaching-fixing solution used. For example, the following methods are known:
(1) a method which comprises adding an oxidizing agent (described, for example, in U.S. Pat. Nos. 3,615,507 and 3,767,401, and German Patent Application (OLS) No. 2,149,314),
(2) a method which comprises contacting the bleaching-fixing solution with oxygen (air) (described, for example, in U.S. Pat. Nos. 3,634,088 and 3,700,450, and German Patent Application (OLS) No. 2,113,651), and
(3) a method involving electrolytic oxidation (described, for example, in Japanese Patent Application (OPI) No. 18191/73).
In oxidizing methods (1) and (2), the sulfite and thiosulfate ions in the solution are oxidized, and unwanted substances and the oxidizing agent added for regeneration in the solution are oxidized, and therefore, unwanted substances are accumulated in the revived solution. Accordingly, these methods require the discarding of the accumulated material and the replenishing of the components of which the amounts are deficient. Furthermore, method (2) cannot be conducted rapidly, and in order to obtain a regenerated bleaching-fixing soluiton having a satisfactory oxidizing capability, long periods of contact time are required. In contrast, in the electrolytic oxidation method (3), the oxidation proceeds without chemical reactions being involved, and therefore, only iron (II) ion advantageously is changed to iron (III) ion. For this reason, the electrolytic oxidation method is preferred.
However, when the electrolytic oxidation method is applied to the recovery of silver simultaneously with the oxidation of iron (II) ion, a reduction in iron (III) ion also occurs simultaneously with the reduction of the silver complex ion in the step of reduction (that is, at the cathode). Thus, a high oxidation capability is required at the anode (if sufficient oxidation is not performed, the regenerated bleaching-fixing solution does not have a sufficient bleaching action). In addition, where an anode is provided with a high oxidation capability, the sulfite and thiosulfate ions are oxidized at the same time. For this reason, it is not commercially acceptable to perform the oxidation of iron (II) ion in the bleaching-fixing bath and simultaneously to recover silver therefrom using the electrolytic method.
In particular, when thiosulfate and sulfite ions are decomposed, addtion of these ions in amounts corresponding to those decomposed must be added to the regenerated bleaching-fixing solution. From an economic viewpoint, this negates the advantage of the regenerating operation, and poses a serious problem. Accumulation of sulfate ions and other substances formed as a result of decomposition causes a retardation of the clearing time, and the sulfur ion formed by the reduction results in the formation of silver sulfide.